The subject matter of this invention relates generally to vacuum circuit interrupters and more particularly to vacuum circuit interrupters having vacuum monitoring devices which utilize internal shields as part of a cold cathode ionization device.
Vacuum type circuit interrupters are well known in the art. Generally a vacuum circuit interrupter is formed by disposing a pair of separable main contacts within a hollow insulating casing, one of the contacts is usually fixed to an electrically conductive end plate disposed at one end of the hollow casing. The other contact is movably disposed relative to another conductive end plate at the other end of the insulating casing. Since a vacuum interrupter requires that the contact region be evacuated, the movable contact is interconnected mechanically with its end plate by way of a flexible bellows arrangement. Typically, the internal portion of the casing is evacuated to a pressure of 10.sup.-4 Torr or less. Because the electric arc of interruption takes place in a vacuum, the arc has a tendency to diffuse and the dielectric strength per unit distance of separation tends to be relatively high when compared with other types of circuit interrupting apparatus. The vacuum circuit interrupter then has a number of significant advantages, one of which is relatively high speed current interruption and another of which is short travel distance for the separating contacts. Since metal vapor is often produced during the interruption process, metal vapor shields are often disposed coaxially within the insulated casing to prevent the vaporous products from impinging upon the inner walls of the casing where the vapor products can condense and render the insulating casing conducting or they could attack the vacuum seal between the electrically conducting end plates and the cylindrical insulating casing. Vacuum type circuit interrupters are shown and described in U.S. Pat. No. 3,892,912 entitled "Vacuum Type Circuit Interrupter" by A. Greenwood et al., U.S. Pat. No. 3,163,734 entitled "Vacuum-Type Circuit Interrupter with Improved Vapor Condensing Shielding" by T. H. Lee, U.S. Pat. No. 4,224,550 entitled "Vacuum Discharge Device with Rod Electrode Array" by J. A. Rich and U.S. Pat. No. 4,002,867 entitled "Vacuum-Type Circuit Interrupters with Condensing Shield at a Fixed Potential Relative to the Contacts" by S. J. Cherry. The latter patent is assigned to the assignee of the present invention. As one might expect the successful operation of the vacuum circuit interrupter requires the presence of a vacuum in the region of interruption. However, if the vacuum interrupter develops a leak so that the gas pressure within the vacuum interrupter rises to a level above 10.sup.-3 Torr, for example, the safe operation of the vacuum circuit interrupter may be seriously hindered if not rendered impossible. Consequently, it has always been a desire to reliably determine whether a vacuum is in fact present in the arc interrupting region. Voltage breakdown apparatus has been utilized as is described in U.S. Pat. No. 3,983,345 entitled "Method of Detecting a Leak in Any One of the Vacuum Circuit Interrupters of a High Voltage Circuit Interrupters of a High Voltage Circuit Breaker" by V. E. Phillips. On the other hand, an oil level measuring system is described in U.S. Pat. No. 3,626,125 by A. Tonegawa. These methods generally are relatively expensive, space consuming and complicated. It was found that the principle of the cold cathode ionization gauge could be utilized relatively simply and inexpensively to detect the presence of a vacuum. Such devices are described in U.S. Pat. No. 4,000,457 entitled "Cold Cathode Ionization Gauge Control for Vacuum Measurement" by C. D. O'Neal III, U.S. Pat. No. 3,582,710 entitled "Ultrahigh Vacuum Magnetron Ionization Gauge with Ferromagnetic Electrodes" by l. J. Favreau and U.S. Pat. No. 3,581,195 entitled "Detection of Vacuum Leaks by Gas Ionization Method and Apparatus Providing Decreased Vacuum Recovery Time" by R. L. Jepsen. A d.c. cold cathode ionization gauge is relatively well known. Simply, it relies upon the spontaneous release of electrons from a "cold cathode" and their subsequent motion under the influence of electric and magnetic fields. The magnetic field has the effect of maintaining the electron in the region between electrodes for a relatively long period of time. It has been found that a self limiting value of 10.sup.+10 electrons per cubic centimeter plus or minus an order of magnitude or so is usually the density of the electron cloud in a typical ion gauge. If a gas is present in the region, the electrons will strike some of the gas molecules, thus causing other electrons to be given off, therefore sustaining the electron cloud. Furthermore, the gas molecules acquire electric charge when impacted by an electron. The charged molecules migrate according to the polarity of the electrostatic field towards one of the electrodes whereupon they each receive an electron from the electrode. As the electrons of the electrode combine with the gas ions at the surface of the electrode to neutralize the ions, an electrical current is sustained in an electrical circuit which includes the electrode. If an ammeter is inserted in series circuit relationship in the aforementioned circuit and calibrated appropriately, an electrical indication of the density of gas present between the electrode is attainable. This principle has been applied to d.c. vacuum circuit interrupters. For example, U.S. Pat. No. 3,263,162 entitled "Apparatus and Method for Measuring the Pressure Inside a Vacuum Circuit Interrupter" by J. R. Lucek et al., and U.S. Pat. No. 3,403,297 entitled "Vacuum-Type Circuit Interrupter with Pressure-Monitoring Means" by D. W. Crouch, teach the utilization of a single shield within a vacuum circuit interrupter utilized in conjunction with one of the main electrodes to form a cold cathode magnetron device. This is made possible by the fact that most of the shields have an intermediate ring which protrudes outwardly through the insulated casing, generally at the axial midpoint of the latter mentioned casing. One disadvantage associated with this type of arrangement lies in the fact that the electron cloud is formed near the main electrode thus enhancing the opportunity for voltage break down between electrodes or electrodes and shield. Another disadvantage lies in the fact that the placement of the magnet around the insulating casing often provides insufficient flux density. Also the formation of the electron cloud near the main contacts often jeopardize the interrupting function. Another cold cathode measuring device is taught in U.S. Pat. No. 4,163,130 entitled "Vacuum Interrupter with Pressure Monitoring Means" by Kubota et al. in which a separate vacuum gauge is attached to an opening in one portion of an end plate of an a.c. vacuum interrupter. This device does not require the presence of the shields or the utilization of the main electrodes directly. However, it creates a disadvantage in that the vacuum integrity of the system must be affected by the mere inclusion of the detection gauge therein. Furthermore because of the geometry of the gauge the pressure inside the device may be different from that in the vacuum chamber. None of the three aforementioned patents teaches the use of multiple shields within the circuit interrupter. It has been shown to be advantageous to use multiple shields within the circuit interrupter as is described for example in U.S. Pat. No. 3,575,656 entitled "Method and Apparatus for Measuring Pressure in Vacuum Interrupters" by W. W. Watrous, Jr. The end shields are spaced from the central shield to maintain the high voltage isolating characteristics. However, the end shields do provide the additional mechanical function of more directly protecting the sensitive end plate to insulating cylinder seal where it is most likely that metal vapors will effect vacuum integrity by destroying the seals. However, in the latter case the internal shield is not available for external circuit connection as it does not protrude through the insulating casing of the circuit interrupter, which did not require no additional penetrations of the vacuum envelope than are already present in the vacuum circuit interrupter because of greater chance of leaks and which use existing vacuum interrupter geometry for reduced cost.